Charlie O'Neill was the only son of ColonelJack O'Neill and Sara O'Neill. He died, shortly before the first mission to Abydos, when he accidentally shot himself with his father's personal gun. Judging from his appearance, he was about eight years old at time of his death.

Jack O'Neill blamed himself for what had happened to his son and came close to committing suicide. However, his adventure on Abydos with Dr.Daniel Jackson brought him out of his depression and caused him to lighten up a little. (Stargate) Nevertheless, Sara divorced her husband shortly after his return from the first Stargate mission. (SG1: "Children of the Gods")

About a year later, in 1997, an alien entity, impersonating O'Neill, tried to find Charlie so he could make him "better," but didn't realize how permanent physical death was to Humans. However, the being did impersonate Charlie as Jack took him back to his planet to give him a chance to say goodbye to his son. (SG1: "Cold Lazarus")

In 2000, when dealing with Malakai, an alien archaeologist trying to make an Ancient time machine work so that he could go back and see his wife again after her death of a congenital heart condition, O'Neill used Charlie's death to show Malakai that he knew the other man's pain and convince him that the pain he would feel from reliving his wife's death wasn't worth the chance of seeing her again. (SG1: "Window of Opportunity")

After invading Jack's subconsciousness mind in 2002, the Human-form ReplicatorFirst made him relive Charlie's death. First told Jack that he was right to blame himself and that Charlie would still be alive if not for his carelessness. (SG1: "Unnatural Selection")

In 2003, eight years after Charlie's death, O'Neill still kept a photograph of Charlie in his locker. (SG1: "Fallen")

In the film, the character's name, as it appeared on a certificate, was Tyler O'Neil. It's been speculated that "Tyler" may be his actual given name and that "Charlie" is only a nickname. Tyler could also be his middle name which was placed on the certificate accidentally. In Bill McCay's Stargate novels, the character was named Jack O'Neil, Jr. after his father.